We are investigating the role of myosin phosphorylation in causing histamine secretion from rat leukemic cells. Upon stimulation, there was a significant increase (2-3 fold) of phosphate incorporation into the myosin heavy and 20 kDa light chains of antigenically primed cells compared to non-stimulated control cells. Maximal phosphorylation was observed at a time (2.5-10 min) when the calcium signal and rate of inositol phospholipid hydrolysis have been shown to be maximal. Two dimensional tryptic peptide maps indicated that the light chains were phosphorylated by myosin light chain kinase (serine -19 site) and protein kinase C (serine -1 or serine -2 site). Whether this phosphorylation of myosin within intact basophils plays a role in the secretory process is under investigation.